1. Field of the Invention
The present invention related to a method of fabricating an SOI (Silicon On Insulator) substrate having a semiconductor layer on an insulation film, which is suitable for a substrate of LSI in next generation, and particularly, a method of fabricating an SOI substrate which is contrived to inhibit a crystal defect in a semiconductor layer.
2. Description of the Related Art
As one method of fabricating an SOI substrate having a semiconductor active layer on an insulation film, there is, for example, a SIMOX (Separation by Implanted Oxygen) method disclosed in Journal of Material Research, Vol. 8, No. 3, pp. 523 to 534, 1993. In the SIMOX method, oxygen ion (O.sup.+) in a high dose, for example, 0.3.times.10.sup.18 to 2.0.times.10.sup.18 (cm.sup.-2), is first implanted into a Si substrate, then, the Si substrate is annealed at high temperature to form a continuous oxide film (SiO.sub.2 film) inside the substrate.
In this method, there is a problem that a crystal defect remains in a Si active layer on the continuous oxide film which is an area on which devices are to be formed, though the SOI substrate can be relatively easily provided. The density of the crystal defects remaining in the Si active layer depends on dose of the implanted oxygen ion, and in the case of high dose, more crystal defects remain as compared with the lower dose case. Therefore, there is suggested a method in which the dose of an implanted oxygen ion is lowered for reducing the crystal defects. However, for example, when an oxygen ion is implanted at low dose, for example, from 3.times.10.sup.17 to 4.times.10.sup.17 (cm.sup.-2), a continuous oxide film is not formed in the following thermal treatment at high temperature, forming a route for a leak current, and excellent device property can not be obtained.
Therefore, there is suggested a method of fabricating an SOI substrate by determining thermal treatment conditions after implantation of an oxygen ion (Japanese Unexamined Patent Publication (Kokai) Nos. Hei 7-201773, 7263538). In the conventional method of fabricating an SOI substrate described in Japanese Unexamined Patent Publication (Kokai) No. Hei 7-201773, a field oxide film for element separation is first formed on a semiconductor substrate, and an oxygen ion is implanted in an area surrounded by the field oxide film. Then, the semiconductor substrate is annealed at 1250.degree. C. or higher in an inert gas atmosphere or the like. By this method, a buried oxide film is formed so as to be continuous with the field oxide film. In this conventional method, the field oxide film promotes the formation of the buried oxide film, and at the same time, a crystal defect caused by the formation of the field oxide film is removed in the following thermal treatment.
On the other hand, in the conventional method of fabricating an SOI substrate described in Japanese Unexamined Patent Publication (Kokai) No. Hei 7-263538, an oxygen ion is implanted into a substrate, then the substrate is annealed, and thereafter, the substrate is again annealed in an oxygen gas atmosphere. FIGS. 1A to 1D are cross-sectional views showing the method of fabricating an SOI substrate described in Japanese Unexamined Patent Publication (Kokai) No. Hei 7-263538, in the process order. In the conventional art, as shown in FIG. 1A, an oxygen ion is first implanted into a single crystal silicon substrate 21, to form an ion-implanted layer 22 in the single crystal silicon substrate 21. Then, as shown in FIG. 1B, an anneal protecting film 23 composed of SiO.sub.2 is formed on the surface of the single crystal silicon substrate 21, using a CVD apparatus. Next, as shown in FIG. 1C, the single crystal silicon substrate 21 is annealed in an inert gas atmosphere, to convert the ion-implanted layer 22 in the single crystal silicon substrate 21 to a first buried oxide film 24. In this process, an oxide film 25 is formed on the surface of the silicon substrate 21. In this point, the thickness of the first buried oxide film 24 is small, pinholes may sometimes be formed in the first buried oxide film 24, and relatively large unevenness may sometimes be formed on the interface between the first buried oxide film 24 and the Si active layer 21a.
Then, the single crystal silicon substrate 21 is annealed at a temperature from 1150 to 1415.degree. C. in atmosphere wherein the oxygen gas concentration is from 1% by volume to 100% by volume. By this treatment, as shown in FIG. 1D, a second buried oxide film 26 is formed, wherein the film thickness is large, and pinhole and unevenness on the interface are reduced. Then, an oxide film 27 thicker than the oxide film 24 is formed on the surface of the silicon substrate 21.
Further, there is also disclosed a method in which the thickness of a Si active layer can be reduced, by annealing the silicon substrate 21 as sacrifice oxidation at 1100.degree. C. or lower, then, removing the sacrifice oxide film thus formed.
However, even by these conventional fabricating methods described in these publications, it is difficult to form a sufficient buried oxide film with inhibiting formation of crystal defects.
Further, a method of fabricating an SOI substrate by adopting a laser recrystallization method is suggested (Japanese Unexamined Patent Publication (Kokai) No. Hei 1-128574). In a conventional method of fabricating an SOI substrate described in this publication, an insulation film is first formed, having opening portions in planned regions for forming devices such as source/drain regions and the like excepting a channel region on a single crystal silicon substrate. Then, a polycrystal silicon layer is buried in these opening portions before another polycrystal silicon layer is laminated on the whole surface. Subsequently, the laminated polycrystal silicon layer is converted to a single crystal by a laser recrystallization method using the single crystal silicon substrate as a seed crystal. Then, an oxygen ion is implanted into the single crystal silicon layer formed in the opening portions and the silicon substrate is annealed, to form a substrate having SOI structure.
By this conventional technique, a crystal defect can not satisfactorily be inhibited though high integration of a semiconductor apparatus can be accomplished.